It seems that when the creator was making the world,he called the man aside and bestowed upon him20 years of normal sex life. MAN was horrified. 'ONLY TWENTY YEARS'
But the creator didn't budge.That was all he would give him.

Then he called the MONKEY and gave him 20 years.
'But I don't want 20 years'.The monkey protestde.Ten is plenty'. MAN spoke up and said 'Can I have the other 10'.
The monkey graciously agrred.

Then he called the LION and gave him 20 years. The LION too only need 10 years. Again MAN said:Can I have the other 10 years'. The lion roared:of course'.

Then came the DONKEY,He was given 20 years,but like the others,10 years was enough. MAN asked for the spare 10 years and got them.

This explains why MAN has 20 years of normal sex life,10 years of monkey around,10 years of lion about it,ans 10 yers of making an ass of himself.